It is known that a piezoelectric ceramic-polymer composite can be produced by mixing ferroelectric ceramic particles of lead zirco-titanate or lead titanate with piezoelectric organic material such as polyvinylidene fluoride, polyvinyl fluoride, polyvinylidene chloride, polyvinyl chloride or nylon, or organic material such as synthetic rubber or synthetic resin, and has acoustic impedance characteristics similar to that of water. It is also known that when such a piezoelectric ceramic-polymer composite material is used for a piezoelectric transducer, it can efficiently receive acoustic waves propagated under water to provide advantages of increasing the sensitivity.
For example, U.S. Pat. No. 4,183,010 discloses a coaxial line hydrophone comprising radially spaced conductors and intermediate dielectric filler material that has both piezoelectric and electret properties such as polyvinylidene fluoride, and method of measuring underwater soundings at either low or high hydrostatic pressures by measuring electrostatically generated signals between conductors at low hydrostatic pressures and piezoelectrically generated signals at high hydrostatic pressures.
U.S. Pat. No. 4,376,302 discloses a piezoelectric polymer hydrophone in which it comprises a single flexible sheet of a piezoelectric polymer having a plurality of electrode stripes on the top and bottom thereof, the electrode stripes at the top are staggered by one half the width of a strip relative to the corresponding strips at the bottom of the sheet and the polymer sheet can be rolled into a helix without losing its acoustic sensitivity.
With the arrangement as mentioned above, since the piezoelectric device is flexible, the piezoelectric portion of the device is subjected to not only an acoustic wave but also a pressure such as, for example, tensile stress at the blowing stream, or a bending stress due to the stream of water or wave. By these mechanical stresses may occur a distortion in the longitudinal direction of the piezoelectric portion, that is, in a direction perpendicular to the polarized axis to generate a charge or voltage which produces a noise signal to be superposed on the acoustic wave, thereby decreasing the S/N ratio.
In order to overcome the problem mentioned above, I have been proposed in U.S. Pat. No. 4,695,988 an underwater piezoelectric arrangement in which it comprises two piezoelectric layers made of piezoelectric ceramic-polymer composites, one of the layers has larger hydrophone constant than that of the other layer, and the piezoelectric layers are so arranged that the charges or voltages generated by stresses which act on these layers in their longitudial direction become substantially equal to each other, thereby producing a difference in the charges or voltages generated on the two piezoelectric layers.
With this arrangement, by cancelling the charges or voltages generated on the both piezoelectric layers by the stresses which act on these layers in their longitudial direction, a noise signal can be removed and thus the S/N ratio can be improved.
However, the previously proposed arrangement has the following disadvantages;
(1) Due to the superimposed structure of the two piezoelectric layers, it is difficult to provide uniformly at high density an adhere layer or insulating layer which should be interposed between the piezoelectric layers and then to avoid an occurrence of air bubbles or blowholes, thereby resulting in a reflection or loss of an acoustic wave, a deterioration of the characteristic and a power reduction.
(2) Since the piezoelectric layers are made of different materials, they are different in a hardness or other physical properties, and then it is difficult that they are formed to have a uniform thickness. Yield in a manufacture of the device may be deteriorated.
It is, therefore, an object of the present invention to overcome the abovementioned drawbacks and disadvantages of the conventional piezoelectric device and to provide an underwater piezoelectric arrangement having a single layer structure.
A further object of the invention is to provide an underwater piezoelectric cable which comprises said piezoelectric arrangement.